The present invention relates generally to temperature standards and more particularly to an improved melting point temperature standard.
It is well known that when a crystalline substance melts, the change of state from solid to liquid requires the input of energy. During the melting process, the energy applied to the crystalline substance goes into producing the change of state rather than into raising the temperature of the substance. Accordingly, while melting continues, the temperature remains constant at the melting point for the crystalline material.
The fixed temperature of the crystalline material during melting can be used as a temperature standard. The best known example is the melting temperature of ice made from pure water which occurs at zero degrees Celsius. Since the melting point of other crystalline substances are at temperatures different from zero degrees Celsius, a number of such melting points may be used as a standard temperature check at temperatures other than zero degrees Celsius.
There has been a need for a temperature reference for use to check electronic, thermistor type temperature probes and the like used in clinical laboratories in the 25 to 37 degrees Celsius range. One such reference has become available with the development by the National Bureau of Standards of a gallium melting point temperature standard, designated as Standard Reference Material 1968. The measuring cell developed by the National Bureau of Standards consists of a plastic cell containing 25 grams of 99.99999 percent pure gallium metal surrounding a well into which a temperature probe can be inserted for calibration. The plastic cell has relatively massive ends with a thin walled central portion through which heat is introduced to melt the gallium. The melting point of gallium is certified by the National Bureau of Standards as 29.7723+0.0004 degrees Celsius (for convenience 29.77 degrees Celsius will be used hereinafter) for a typical standard. When the cell is located in a bath having a temperature slightly above the melting point so that the gallium melts slowly, the certified temperature can be maintained in the well until the gallium has melted. Once all the gallium has melted, it must be refrozen prior to using the cell for further calibration cycles.
The National Bureau of Standards cell must be raised to a temperature slightly above the gallium melt point in order to produce the reference temperature within the well. This can be achieved by immersing the cell in a temperature controlled bath, however, such a procedure does involve equipment that is not only bulky but also quite costly.
Therefore, it is an objective of the present invention to provide a means for utilizing a melting point cell as a standard which is small, relatively low in cost, requiring no adjustments and which will extend the useful length of the melting cycle over a normal working day without the use of a bulky/costly liquid bath.
It is still a further objective of the present invention to provide a melting point cell with a built-in means for rapidly refreezing the crystalline substance after the melting cycle so it can be repeated.
It is yet a further objective of the invention to provide an automatic and continuous indication of the state of the crystalline material in a melting point cell during the melting cycle whereby, when the gallium becomes completely melted and is no longer maintaining a proper reference temperature, a visual or other indication will apprise the user of that fact and, if desired, initiate the refreezing cycle automatically.